Primordial germ cells (PGCs) are the founder cells of the germ line, and are the only totipotent cells remaining in a developing embryo. It is important to understand how these cells develop, and how the totipotent condition is maintained during development, not only because these cells give rise to adult gametes, but also because they are one of the only sources for the production of cultured embryonic stem cells. The PGCs are a small transient population of cells, and are difficult to obtain as raw material for experiments. As a result very little is known about how these cells develop, particularly at a molecular level. Very few genes whose expression is restricted to the PGCs have been isolated. A battery of such genes will be required in order to understand the molecular mechanisms that govern PGC development. To address this a cDNA library was constructed from the dorsal mesentery of Xenopus embryos, at a time when this structure contains all of the PGCs. Experiments by Wylie et al. (1985) demonstrated directly that PGCs in the dorsal mesentery are totipotent, and therefore the genes required to maintain the totipotent condition must be present in the library. As a first attempt to isolate novel germ cell specific genes the dorsal mesentery library was screened with subtracted cDNA probe derived from Xenopus oocytes, the adult female germ cells, making use of the fact that many genes expressed in PGCs are also found in oocytes. The screen identified 76 independent clones that react under stringent conditions with probe from oocyte, but do not react with probe from liver, which is a representative somatic tissue. Work is proposed to identify the sequences encoded in these clones and test their expression in both adult and embryonic tissue. The results of this work will provide the raw materials to pursue later studies aimed at understanding the function of these genes during the development of the Xenopus germ line.